1. Field of the Invention
The present invention relates to a sheet processing apparatus provided in a main body of an image forming apparatus such as a copier, a printer or the like and adapted to process a sheet sent from the main body of the image forming apparatus, and, more particularly, it relates to a sheet processing apparatus which can store sent sheets while the sheet is processed.
2. Related Background Art
In recent years, as an option for an image forming apparatus such as an electrophotographic copier, a laser beam printer or the like, a sheet processing apparatus such as a sorter for sorting imaged sheets has been developed. Such a sheet processing apparatus performs at least one of sorting processing, stapling processing and aligning processing operations with respect to the sheet.
In a sheet processing apparatus having a stapler for performing the stapling processing, after sheets conveyed into a main body of the sheet processing apparatus are stacked on a processing tray through a conveying path provided within the main body, the stapling operation is performed.
In the sheet processing apparatus for stapling a sheet stack (or a bundle of sheets), the sheets are stacked on the processing tray as a sheet stack and the sheet stack is stapled at one position or plural positions (normally, two positions) while shifting the stapler as stapling means. While the stapling operation is being performed, sheets for a next job cannot be stacked on the processing tray. Thus, a sheet-to-sheet distance between the successive jobs for the stapling operation must be lengthened.
However, if the sheet-to-sheet distance is lengthened, productivity will be worsened. That is to say, the number of sheets to be processed per unit time will be reduced. In order to prevent the reduction in the productivity, there has been proposed a sheet processing apparatus in which a sheet holding portion (buffer portion) for storing sheets is provided in a conveying path for conveying the sheet to a processing tray.
In such a sheet processing apparatus, while plural sheets stacked on the processing tray are being processed, plural succeeding sheets are stored in the sheet holding portion, and, when the processing is finished, the sheets stored in the sheet holding portion are stacked on the processing tray and the succeeding sheets are supplied onto the processing tray until a desired number of sheets are stacked.
There are two kinds of sheet processing apparatuses having such a sheet holding portion. As a first kind, there is a sheet processing apparatus of sole stack discharging type in which, when the processing of the sheet stack on the processing tray is finished, the sheet stack on the processing tray is discharged and then the sheets stored in the sheet holding portion are discharged onto the processing tray (for example, refer to Japanese Patent Application Laid-open No. 9-48545 (FIGS. 1 and 2)). As a second kind, the Applicant proposed a sheet processing apparatus of simultaneous stack discharging type in which, when the processing of the sheet stack on the processing tray is finished, an operation for discharging the sheet stack on the processing tray from the processing tray and an operation for discharging the sheets stored in the sheet holding portion onto the processing tray are performed simultaneously. However, this sheet processing apparatus of simultaneous stack discharging type was not laid-opened on the priority date of this application and was laid-opened later (as Japanese Patent Application Laid-open No. 2003-81517).
However, the two kinds of conventional sheet processing apparatuses had the following problems.
In both the sheet processing apparatus of sole stack discharging type and the sheet processing apparatus of simultaneous stack discharging type, if a sheet is jammed at an upstream side of the sheet holding portion while the sheets are being stored in the sheet holding portion, since the entire apparatus is stopped, the sheet stack stacked on the processing tray and the sheets held in the sheet holding portion remain within the apparatus, and, thus, a jam recovering time is greatly increased, thereby worsening processing efficiency.
Further, in the sheet processing apparatus of simultaneous stack processing type as the second kind, in a case where the sheet stack is stacked on the processing tray and the sheets are being stored in the sheet holding portion, for example, if adjustment such as color adjustment is performed before a predetermined number of sheets are sent to the sheet holding portion, a succeeding sheet may be sent in a delayed manner. In such a case, while the succeeding sheet is being sent in the delayed manner, the processing of the sheet stack on the processing tray may be already finished.
However, in the sheet processing apparatus of this type, after the number of sheets sent during the processing of the sheet stack on the processing tray are held in the holding portion, since the simultaneous stack discharging operation in which the stored sheets are discharged onto the processing tray and, at the same time, the sheets stacked on the processing tray are discharged is performed, so long as the predetermined number of sheets are not sent to the holding portion, even if the processing of the sheet stack on the processing tray is finished, the sheet stack cannot be discharged outside. Thus, the sheet processing apparatus of simultaneous stack discharging type has poor processing efficiency.